The Challenge Card SystemAfter purchasing a challenge card (from an NPC found next to the one who sells pots), you can queue for a specific boss. This is a diversion from the rank progression and has its own separate achievement, Now You're Just Showing Off (Season 2).

The old system of discovery is still in place as well. Accessing these bosses requires going out into the world and "discovering" quest items that unlock them. Check the page for Deck Your Collection (Season 1) for sources. At this time, I'm not sure what ranks each boss requires.